Sulphur has 16 electrons about its nucleus. These orbit around at different energy levels, called shells, the innermost shell being the most tightly bound to the positively charged nucleus. Two electrons orbit around the nucleus in the first shell, 8 electrons orbit in the second shell, and only 6 orbit in the third shell. All atoms tend to react to complete their outer shells; the third shell would like to contain 2 more electrons for a total of 8, and it does this by sharing the electrons of another atom. This is why sulphur reacts with other atoms willing to share their surplus electrons and form compounds.